Magnetic resonance imaging (MRI) is a medical imaging technique in widespread use for viewing the structure and/or function of the human body. MRI systems provide soft-tissue contrast, such as for diagnosing soft-tissue disorders. MRI systems generally implement a two-phase method. The first phase is the excitation phase, in which a magnetic resonance signal is created in the subject with a main, polarizing magnetic field, B0, and a radio frequency (RF) excitation pulse, B1+. The second phase is the acquisition phase, in which the system receives an electromagnetic signal emitted as the excited nuclei relax back into alignment with the main magnetic field after the excitation pulse B1 is terminated. These two phases are repeated pair-wise to acquire enough data to construct an image.
The recovery period to allow for spin relaxation may lead to relatively slow acquisition times. The sequential nature of the scanning process also lengthens acquisition times. MR imaging accordingly often involves the acquisition of two-dimensional (2D) slices rather than three-dimensional (3D) volumes. The number of encoding steps is lower in 2D imaging, because one encoding dimension is omitted. The acquisition time for a 2D image is just a fraction of the time for a 3D image.
Newer-generation MRI systems transmit multiple radio-frequency pulse trains in parallel over independent radio-frequency transmit channels, e.g., the individual rods of a whole-body antenna. This method, referred to as “parallel transmission” or “parallel excitation,” reduces acquisition times.
Reduced acquisition times and other advances enable more dynamic MRI procedures directed to studying the motion of an object. Dynamic MRI is used to provide cine imaging of the heart. The reduced acquisition times maintain image quality despite the reduced number of gradient encodings. Cardiac MR imaging is typically conducted at different planes or slices, with a separate 2D acquisition at each target plane. The heart is covered through a stack of 2D slices instead of a single 3D acquisition.
Conventional multiple-slice 2D cine acquisitions lack significant details due to large gaps between slices. The number of slices is limited by the measurement time. Moreover, misalignment of the slices arises due to extraneous motion. In cardiac imaging, artifacts may be present due to respiratory motion. Patients are asked to hold their breath repeatedly during scanning, which poses a difficult challenge for some patients.